1. Field of the Invention
The present invention relates to an optical disk unit that records and reads information to and from an optical disk, and particularly to an optical disk unit that can accurately measure the relative speed of a light spot radially moving on the optical disk to access a target track on the optical disk.
2. Description of the Prior Art
An optical disk unit employs a light spot to access one of a plurality of tracks formed on an optical disk. This track accessing operation is generally carried out by a combination of speed control and position control.
In speed control, a relative speed of the light spot radially moving on the optical disk by crossing the tracks is measured, and compared with a reference speed to provide a speed error signal. Based on the speed error signal, the speed of the light spot is controlled.
When the light spot reaches a target track, the position control is started to make the light spot follow the target track. In position control, a positional error signal is provided based on an error between the position of the light spot and target track. According to the positional error signal, the position of the light spot is controlled.
FIG. 1 shows a movement of the light spot on the optical disk. To access a target track where required data is recorded, the light spot is moved from a present track to the target track according to the speed control at first. When the light spot reaches the target track, the light spot is slowed down, and the speed control is switched to the position control.
To measure a relative speed of the light spot on the optical disk for the speed control, a conventional technique detects a signal from the optical disk, and generates a pulse whenever the light spot crosses one or a half track. An interval of pulses thus obtained corresponds to the relative speed of the light spot. The conventional technique measures a time period of the interval, and executes arithmetic operations including a multiplicative inverting operation to find the relative speed of the light spot.
FIG. 4 shows a principle of the conventional speed measuring technique. In the figure, a curved line indicates temporal changes of an actual relative speed of the light spot, and a straight line a measured relative speed of the same.
Pulses whose intervals change in response to the relative speed of the light spot are generated at t1, t2, and so on, as the light spot is moved on the optical disk. When a pulse is generated at t1, a time measurement starts. When the next pulse is generated at t2, an elapsed time (t2-t1) for the period from t1 to t2 is provided and held. An inverse number of the elapsed time for the period of (t2-t1) is calculated and multiplied by a proper coefficient to provide a speed d2, which is an averaged speed of the light spot moved relative to the optical disk for the period of (t2-t1). The speed d2 is held for a period from t2 to t3. This operation is repeated for each pulse generation to determine the relative speed of the light spot on the optical disk.
The technique mentioned above is relatively easy to provide the relative speed of a light spot. The technique has, however, several drawbacks. As shown in FIG. 4, the measured speed indicated satisfactorily follows the actual speed only before t4. After t4, the measured speed greatly deviates from the actual speed. The reason for this is because the measured speed is not updated frequently in a deceleration state where an interval of pulses gradually gets longer. In an extreme case of beyond t7, a speed d7 measured at t7 is maintained forever, if no pulse is generated after t7, even when the actual speed becomes 0 after t7. If such an incorrect measurement is used for controlling the light spot, a phase delay may occur which destabilizes a motion of the light spot.
One technique for providing information such as the relative speed of a light spot for various control purposes is a sample servo technique, which will be explained with reference to FIGS. 2 and 3.
An optical disk 1 shown in FIG. 2 has spiral or concentrical tracks 2. Servo areas 3 are intermittently formed on the tracks 2 to provide detection signals from which information for various control purposes is obtainable. FIG. 3 shows an example of the servo area 3. The servo area 3 of each track 2 involves wobbled pits 7 and 8 disposed on the left and right sides of the track, an access mark portion 6, a mirror portion 4, and a clock pit 5. The wobbled pits 7 and 8 provide tracking signals, and the mirror portion 4 provides a focus error signal. The clock pit 5 provides a system clock to be used for recording and reading operations.
The access mark portion 6 provides a pit pattern that is specific for predetermined tracks and is utilized to detect the number of tracks crossed by a light spot radially moving on the optical disk 1, in a target track accessing operation. FIG. 3 shows 16 pit patterns that repeat every 16 tracks.
FIG. 5 shows a conventional optical disk unit for achieving the sample servo technique explained above. In the figure, a laser beam source 10 emits a laser beam, which passes through a collimator lens 11, beam splitter 12, and objective lens 13, and the beam spot is incidence upon an optical disk 14. A reflection of the light spot from the optical disk 14 is passed through the objective lens 13, reflected by the beam splitter 12, and led to a photodetector 15.
The photodetector 15 provides an output signal 16, which is amplified by a preamplifier 17, and led to a read signal processing circuit (not shown) and a waveform shaping circuit 18, which provides a binary signal 19. The signal 19 is transferred to a clock generator 20, which regenerates a system clock 21 based on a clock pit of a servo area of the optical disk 14.
A timing signal generator 22 uses the system clock 21 to provide a timing signal 23 to be used for detecting an access mark from the binary signal 19. A crossed tracks number detector 24 uses the timing signal 23 to detect the access mark from the binary signal 19, and provides a crossed tracks number 25 when a change is detected in the access mark. The crossed tracks number indicates the number of tracks crossed by the light spot. The crossed tracks number 25 is supplied to a storage circuit 27 through a latch circuit 26, and multiplied by a coefficient to provide a speed data signal 28. The signal 28 is converted by a digital-to-analog converter 29 into a speed signal 30.
FIG. 6 shows relative speed characteristics measured by the conventional optical disk unit explained above. In the figure, an abscissa represents actual speeds, and an ordinate measured speeds. Values shown in the figure are based on a period of appearance of the servo areas, i.e., a sample period, of 20 .mu.s and a track pitch of 1.6 .mu.m. An actual speed in the range of 0 to 0.08 m/s will be measured as 0 or 0.08 m/s, and an actual speed in the range of 0.08 to 0.16 m/s will be measured as 0.08 or 0.16 m/s. The reason of this is because resolution for the access marks is one track to cause a maximum error of 0.08 m/s.
A reading operation of the access marks in the servo areas by the conventional optical disk unit will be explained with reference to FIGS. 7a, 7b, and 7c.
FIG. 7a shows access marks of tracks A and B, FIG. 7b output signals based on the access marks of FIG. 7a, and FIG. 7c binary signals shaped from the output signals of FIG. 7b. The optical disk unit is storing sixteen kinds of reference binary patterns with which a detected pattern is checked to see whether it matches one of the reference binary patterns.
Supposing the binary pattern Qa of FIG. 7c is received, it is recognized that a light spot is on the track having the access mark A because the received signal matches one of the stored patterns. By comparing access marks read at two consecutive sample times with each other, the number of tracks crossed by the light spot during the sample period will be detected. For example, it is detected that the crossed tracks number is one from the access marks A and B. In way, if a pattern that matches one of the 16 reference access mark patterns is detected, the number of tracks crossed by the light spot can be detected.
The light spot on the optical disk has a certain size, and therefore, signals such as a and b shown in FIG. 7b are sometimes detected between the tracks A and B, although the detected level thereof is low.
If such signals are obtained between the tracks, the signals may provide a combined pattern a-b or Qa-b shown in FIGS. 7b and 7c. Since the combined pattern does not match with any one of the stored patterns, it is judged that a read error has occurred, and no operation is carried out to find the crossed tracks number. Namely, a correct crossed track number will not be obtainable until a matching pattern is detected. This may reduce the chance of obtaining information for controlling the movement of the light spot, and lead to an insufficient control of the accessing operation.
The conventional optical disk unit has another problem of providing pattern signals alternately. When output signals from an optical disk are unstable or when tracks of the optical disk are eccentric relative to the center of the optical disk, binary pattern signals such as Qa and Qb of FIG. 7c may alternately be detected even when the light spot is stationary between the tracks A and B. This may happen during an accessing operation in which the light spot crosses a track in several sample periods, and particularly in a latter half of the accessing operation. If this happens, it is judged that the light spot is moving irregularly back and forth on the optical disk, and then, the light spot which is actually moving smoothly in one direction will wrongly be controlled.
As explained above, the light spot on the optical disk has a certain size, and therefore, detects signals between adjacent tracks of the optical disk. This may cause the following problems:
(1) A combined pattern such as Qa-b of FIG. 7c, if detected, is judged as a read error, and the optical disk unit carries out no operation of finding a crossed tracks number. This reduces a chance of correct control of the light spot, and destabilizes the movement of the light spot on the optical disk.
(2) Alternate detection of binary patterns from adjacent tracks also destabilizes the movement of the light spot on the optical disk, because the optical disk unit will judge from the alternate detection that the light spot is moving alternately in forward and backward directions, and unnecessarily control the movement of the light spot according to the judgment, even when the light spot is moving smoothly in one direction on the optical disk.